This invention relates to the field of communication protocol for facilitating communication between controllers of a nodal network suitable for real-time operations control.
Historically, controller communication networks have been organized in star, multi-drop, or ring configuration. In a star configuration, one controller, or "node", forms a center and acts as the network "master". Separate line extend from this master to all the other "slave" nodes. As a result, messages generated by the sending slave node and intended for another slave node must be received by and transmitted to the intended receiving slave node by the master node. Because all inter-nodal communications must go through the master node, thereby consuming master node processor time, star network configurations are suited for those applications which required limited inter-nodal communications of relatively short data messages, which is not a characteristic of networks designed to control real-time interdependent machine operations. Thus, protocols designed to operate within star configured networks are more concerned with data message compression and complete data messaging, and are not well suited where frequent communication inter-nodal communication is expected or where the inter-nodal communication is expected to comprise "statement-response" messages or where large data messages are anticipated.
In the multi-drop network, a communication line, or data trunk, connects to a plurality of nodes by drop lines. A single node may act as a network master, mastership may be transferred between nodes after a polling process determines which node requires ccntrol of the network. Also, in some multi-drop networks, contingent schemes are employed to avoid the need for identifying a master. In such systems, each node contends for assess to the data trunk, and when assess is obtained, a message may be sent directly to any other node in the network. Special hardware is required to detect collision of message on the data trunk. Protocols designed for multi-drop configuration must conform to the requirement of collision detection hardware and bus availability schemes. In addition, the protocol best serves the multi-drop network when the data messages count is somewhat larger as contracted with the star network configuration, however, inter-nodal communication should be infrequent. Again, multi-drop network communication protocols are not particularly well suited for high speed "statement-response" messaging.
With respect to a ring configuration, each node is linked to two other nodes in a loop arrangement. Messages are removed from the ring by the destination node, or if the message travels full circle, it is removed by the original node. Because all communication is lost if any node fails in the ring network, it is common practice to revive a second, or redundant ring which is switched into operation if the primary ring fails. Contrasted with the star and multi-drop networks, the ring configuration is better suited for the communication of high count data messages. However, because of its configuration, it is not particularly suited of high speed "statement-response" messaging. Resultantly, protocols primarily intended for employment in ring networks are designed to promote source and destination nodal identity, and data message delivery.